A conventional lens is incapable of accurately portraying objects that are off axis to the lens or at different distances from the lens. These perspective distortions can create significant problems for optical systems such as, for example, interpretation of data by computer software. A telecentric lens, however, maintains a constant viewing angle at any point across the clear aperture of an associated objective lens. The telecentric lens can therefore be used to accurately portray objects that are slightly off axis or at different distances from the lens. The ability to accurately portray objects in the manner described makes the telecentric lens particularly suitable for use in measurement devices for manufacturing processes. By eliminating perspective distortion, the telecentric lens produces a dimensionally accurate image, which is simple for software to interpret.
In prior art telecentric lenses, the refractive elements are formed by materials such as glass or plastic for visible light; and other materials are employed for use in the infrared region of the electromagnetic spectrum. For any particular type of material used, however, the useable spectral range is greatly limited. Also, as is the case for all imaging devices, as the required size of the device is increased, the weight, complexity, and cost are increased in order to maintain reasonable performance. As the size of refractive imaging devices increase, the imaging devices become problematic more quickly than equivalent performance reflective devices, once a certain size threshold is reached. Such inherent problems are the reasons that larger optical telescopes are made from concave mirrors (reflector telescopes) as opposed to glass elements (refractor telescopes). Also, in general, reflective imaging devices require far fewer optical elements than refractive devices of the same general performance.
Telecentric lenses of the type used to form images of close terrestrial objects, especially at magnifications that are more useful for industrial imaging such as automated inspection or machine vision, have been limited to all-refractive designs. One of the reasons for this limitation is that imaging of near or terrestrial objects can suffer greatly from having a portion of the field of view blocked by a central transfer mirror or lens. The transfer mirror or lens is used with near impunity for astronomical telescopes used for viewing objects that are nearly infinitely far away and thus remain relatively unaffected by the central blocker. U.S. Pat. No. 4,798,450 to Suzuki discloses other multiple mirror designs that overcome the central blocker problem for terrestrial objects, and consist of special and expensive mirror forms such as off-axis parabolas, ellipsoids, or hyperboloids.
An object of the invention is to produce a reflective telecentric lens that may be constructed less expensively than equal performance all-refractive telecentric lenses.
Another object of the invention is to produce a reflective telecentric lens that may be constructed less expensively than multi-mirror reflective telecentric lens designs.
Another object of the invention is to produce a reflective telecentric lens having fewer components than equivalent performance devices of prior art.
Another object of the invention is to produce a reflective telecentric lens that may be easily re-configured to accommodate different magnifications.
Still another object of the invention is to produce a reflective telecentric lens that has improved optical performance.